'Hearing something through the grapevine', means when you hear about something indirectly, people gossiping and stuff like that. He is asking her, why she didn't have the guts to tell him, herself, instead of letting him find out about her, cheating on him, from someone else.
'Hearing something through the grapevine', means when you hear about something indirectly, people gossiping and stuff like that. He is asking her, why she didn't have the guts to tell him, herself, instead of letting him find out about her, cheating on him, from someone else.